[Octave-bug-tracker] [bug #45565] Using imwrite to write an 8-bit iamge to BMP creates a 24 bit image

[Mike Miller]

Follow-up Comment #16, bug #45565 (project octave):

So I analyzed the BMP header fields more closely and found that Octave creates
a BMP using RLE compression by default, while the file you have attached that
you say works is not using any compression. This agrees with the info I saw
from PIL.

So my simple thought at this point is that Octave needs to tell GraphicsMagick
to write uncompressed BMP files, if GraphicsMagick exposes that knob and
allows a way for Octave to set that. This seems like a Matlab compatibility
item, based on https://www.mathworks.com/help/matlab/ref/imwrite.html saying
that BMP files are always uncompressed.

This is independent of whether imwrite with a uint8 argument should write
8-bit BMP files by default or whether it requires a colormap argument. I can't
decipher Matlab's help page for imwrite to figure out if that's a separate
compatibility issue.

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